This invention relates to a flue gas desulfurization process and an apparatus therefor. More particularly it relates to a wet type flue gas desulfurization process by removing sulfur oxides (SO.sub.x) contained in flue gases, with limestone (CaCO.sub.3), quick lime (CaO) or slaked lime (Ca(OH.sub.2)) as an absorber, and at the same time recovering gypsum (CaSO.sub.4.2H.sub.2 O) as byproduct.
Wet type flue gas desulfurization processes have been directed to a process of converting sulfur oxides SO.sub.x (usually SO.sub.2 or SO.sub.3) contained in flue gases, into gypsum which is chemically stable and utilizable, with CaCO.sub.3, CaO or Ca(OH).sub.2 as raw material, which is abundantly existent in nature and cheaply available, to make flue gases harmless.
Conventional flue gas desulfurization processes comprise a cooling step, an absorption step, a concentration step and a flue gas heating step. Namely, flue gas is first introduced into the cooling step where it is partly desulfurized, at the same time cooled and dedusted and then sent to the absorption step where it is contacted with a slurry containing limestone and lime (which slurry will hereinafter often be referred to as absorber slurry or limestone slurry) and desulfurized therewith and then sent to the flue gas heating step where it is heated and exhausted. The slurry containing limestone and lime absorbs sulfur oxides in the absorption step and byproduces calcium sulfite (CaCO.sub.3) and calcium hydrogen sulfite (Ca(HSO.sub.3).sub.2), but since the slurry partly contains unreacted CaCO.sub.3 and slaked lime (Ca(OH).sub.2), it is recycled to the cooling step where the amounts of CaCO.sub.3, etc. are reduced. Further, a part of CaSO.sub.3 and Ca(HSO.sub.3).sub.2 as byproducts is oxidized in the absorption step and the cooling step to form gypsum. The slurry used in the cooling step is sent to the concentration step. Water separated therein is reused, while concentrated solids (CaSO.sub.3, Ca(HSO.sub.3).sub.2 and CaSO.sub.4) are discarded into a pond adjucent to the step. Further, after the solids are precipitated, the resulting water may often be reused.
However, according to the above-mentioned conventional processes, the byproducts consist of a mixture of CaSO.sub.3, CaCO.sub.3 ; hence they, even if recovered, contain a large amount of byproducts so that it is impossible to effectively utilize them. Further, even in the concentration step, since CaSO.sub.3 and Ca(HSO.sub.3).sub.2 are existent in large amount, particles have particle diameters as small as several .mu.m and also are inferior in settlability; hence as the concentration apparatus, the apparatus capacity such as that of the settling tank is inevitable to be enormously enlarged. As mentioned above, various problems have been raised. Furthermore, in the case where a slurry of calcium compounds is used as an absorber, calcium sulfite and calcium sulfate as byproducts are a main cause of scaling; hence the problem of scaling is inevitable and requires countermeasures.
From such a viewpoint, there has been desired a process by which gypsum can be recovered as byproduct in accordance with its uses, the amount of water used therein is small, effluent treatment can be carried out in a closed system, no scaling occurs and and reliability is high. Further, in this case, from a viewpoint of process efficiency, a process using an apparatus as minimal as possible has been desired.
An object of the present invention is to provide a process which is able to recover different qualities of gypsum corresponding to its uses in a flue gas desulfurization system.
Another object of the present invention is to provide a desulfurization process by which the amount of water used is small as well as a process by which effluent treatment can be carried out in a closed system.
Still another object of the present invention is to provide, from a viewpoint of process efficiency, a flue gas desulfurization process which enables the apparatus to be used as minimal as possible, has a high reliability, and is commercially superior.
The other objects of the present invention will be apparent from the following description.